Patterns in a dual ring.

Terms

a(0) =1a(1) =3a(2) =7a(3) =15a(4) =31a(5) =60a(6) =113a(7) =207a(8) =373a(9) =663a(10) =1167a(11) =2038a(12) =3537a(13) =6107a(14) =10499a(15) =17983a(16) =30703a(17) =52272a(18) =88769a(19) =150407a(20) =254321a(21) =429223a(22) =723167a(23) =1216490a(24) =2043361a(25) =3427635a(26) =5742463a(27) =9609327a(28) =16062463a(29) =26821668